How to Maintain Stones – “Polishing” is the Key?

In people’s minds, it is often believed that stone is a long-lasting high-end decorative material, and the use of stone decoration can be achieved once and for all. Actually, in the natural environment, dust, exhaust gas, acid rain, freezing, and other pollutants can easily cause fading, pollution, and even damage to the stone used in buildings, greatly reducing the decorative effect and service life of the stone. At the same time, due to errors in construction methods, the failure of stone decoration can also result in the need to renovate the building in a short period of time, resulting in a significant waste of manpower and resources.

01. Main reasons for affecting the service life of stone and causing stone diseases

Common stone diseases mainly include:

Non drying of water spots, salt precipitation and alkalinity (white bloom), rust spots and yellowing, frost damage, surface corrosion, pigment pollution (including oil stains), moss growth and powdering peeling, etc. There are many reasons for various pathological changes and shortened service life of stone, which can be summarized into two main aspects:

1. The internal reasons of the stone, namely the changes caused by the structural structure and chemical composition of the stone;

(1) Structural Structure of Stone

There are micro cracks and fine pores in natural stone, which can adsorb fine dust and dirt from the air over time. Due to capillary action, they slowly penetrate into the interior of the stone, greatly reducing its decorative performance and service life. The smaller the pore size, the stronger the capillary effect and surface adsorption.

(2) Relatively complex chemical composition:

Some components in stone are prone to interact with the external environment, causing erosion or variation of the stone. Stone materials generally contain iron to varying degrees, and can produce rust yellow due to moisture erosion. The higher the iron content in stone, the easier it is to produce rust yellow. In addition, most natural marble is mainly composed of carbonate, such as calcium carbonate and magnesium carbonate, and a small amount of basic oxide. Calcium carbonate and magnesium carbonate are relatively stable compounds, but the chemical properties of some basic oxide are unstable. In a humid environment, they are easy to react with acid oxides in the air, such as carbon dioxide, sulfur dioxide, sulfur trioxide, nitrogen dioxide, etc., to change the physical structure of marble.

Similar chemical damage can also occur on other stones, with light cases causing the surface of the stone to lose its luster and even fade in color, resulting in spots; Heavier ones can cause a decrease in the hardness of natural stone and cause surface weathering and peeling.

2. The external factors of the stone, such as mining and processing methods, installation and construction processes, and usage environment, have an impact.

There are many external factors that affect the decorative effect and service life of stone, such as improper maintenance during mining, processing, storage, and transportation, the influence of bonding materials used for stone installation, and natural environmental factors.

(1) The impact of mining, processing, and storage and transportation processes:

▶  Cracks in stone materials may be related to improper mining methods;

▶  The iron tools used for processing may also cause rust and yellowing of the stone, leaving a hidden danger. Improper stone cooling fluid may contaminate the stone;

▶  Improper maintenance during storage and transportation can also cause rust spots and yellow grass rope pollution on the stone.

(2) The impact of installation construction technology and bonding materials:

If the installation of stone is carried out using the wet bonding method of cement mortar bonding, the alkaline soluble substances in the cement will seep out along the pores of the stone due to rainwater erosion, and alkaline compounds such as Ca (OH) 2 will react with CO2 and H2O in the air, generating CaCO3 to crystallize on the surface of the stone, forming a phenomenon of salt precipitation and alkalinity.

In addition, due to the hygroscopicity of certain salts generated after cement hydration, they can absorb moisture from the air and form water spots on the surface of the stone, seriously affecting its aesthetics. In addition, improper use of bonding materials, such as certain stone bonding adhesives, may also cause yellowing of the stone.

(3) Impact of environmental factors:

Environmental factors include environmental pollution, acid rain erosion, and freezing damage. Polluted air, acid rain, and other anthropogenic pollutants in the natural environment can cause pollution and corrosion of stone materials. In addition, freezing damage is also an environmental factor that cannot be ignored, as stone materials have water absorption properties (even when dry hanging construction is used). In cold winter, freezing accelerates their weathering process, reducing their strength, not only affecting their service life, but also threatening personal safety.

02. Visible Lesions and Classification of Stone Materials

As long as all stone materials are exposed to the atmosphere, they will undergo pathological changes. In order to protect historical and cultural heritage, especially natural stone related historical and cultural heritage and modern architecture, the following classification is made based on the effects of various pathological changes on stone materials, and the relevant definitions of each form of pathological changes are indicated.

1. Condition deterioration category

(1) Color change: Natural changes occur in the composition of the stone in the parameters that determine color. Generally, it will extend to the entire rock, and if the change is local, it is called a stain.

(2) Spot: A specific natural component of a material or a localized coloring of an external material on the relevant surface.

(3) Copper green: A natural variation on the surface that is unrelated to a special phenomenon and makes people feel that the original color of the material has changed.

2. Material loss category

(1) Erosion: In most cases, it is the removal of material from a dense surface.

(2) Differential erosion: Clearly exhibiting unevenness in stone composition or typical structure.

(3) Pitting and erosion: forming a large number of adjacent blind holes. The holes are mainly hemispherical in shape, with a maximum diameter of a few millimeters.

(4) Erosion and honeycomb decay: forming hollow grooves with varying sizes, also known as small bubbles and pits, often interconnected and unevenly distributed.

3. Morphological damage category

(1) Decomposition: The material forms powdery or very small debris that falls off and peels off.

(2) Flaky peeling: The formation of one or several thin sheets of very small thickness.

(3) Peeling: It is often caused by the continuous decomposition of the original material, resulting in the separation of irregularly shaped and unevenly thick thin sheets.

(4) Separation: The continuous decomposition between the layers on the surface of a material, or between the upper and lower layers, generally indicates that these layers will fall off. On stone, the separated parts often have specific shapes based on their structural and grain characteristics, such as peeling, scaling, and crusting.

(5) Loss, missing: 3D component missing

(6) Voids and pores: mainly missing parts with the development of two dimensions

4 Secondary products

(1) Crust and secondary hard shell: generally, the density increases with limited expansion, and the increasing direction is parallel to the surface and perpendicular to the surface, the latter will have stalactite or stalagmite shape.

(2) Surface deposition: The deposition of foreign materials of various properties, such as dust, loose soil, and bird droppings. Generally speaking, it will not adhere to the underlying material.

(3) Hardskin: Modification of the surface layer of stone. The thickness varies, usually hard, and differs greatly from the underlying layer in terms of morphological characteristics, especially in terms of color. When the bottom layer undergoes weathering or powdering, the surface layer will separate.

(4) Weathering: The formation of salt on the surface, usually in a white and crystalline or powdery or filamentous form.

(5) Thin film, coating: A transparent or semi transparent surface layer that combines non stone materials together

(6) Biological hard shell: A homogeneous thin layer composed almost entirely of microorganisms, with variable density and color, and combined with the bottom layer according to environmental conditions.

(7) Bioparasitism: The bottom layer of plant-based organic matter visible under a microscope

5 Strength reduction category

(1) Deformation: The appearance or shape of a material changes throughout its thickness.

(2) Bulging: Local surface lifting of the material, with variable shape and density.

(3) Fracture or crack: The continuous decomposition of materials that cause local displacement.

(4) The following is a list of which types of lesions are prone to occur on which types of stones.

▶ Color changes: colored sedimentary rock, igneous rock and green metamorphic rock;

▶ Stain: metamorphic rock with iron sulfide texture;

▶ Copper green: metamorphic rock;

▶ Erosion: sedimentary rock, metamorphic rock with carbonate composition;

▶ Differential erosion: fine bedded or fine textured sedimentary rock, textured metamorphic rock;

▶ Dents and honeycombs: sedimentary rock with uneven structure;

▶ Decomposition and pulverization: all rocks, especially igneous rock hydrolyzed by silicate, clastic sedimentary rock dissolved by cementation agent, and some metamorphic rock dissolved by calcite crystals;

▶ Scale falling: fine bedding or thin sedimentary rock, metamorphic rock with very developed schistosity;

▶ Spalling: metamorphic rock, chemically separated dense sedimentary rock;

▶ Separation: metamorphic rock, clastic sedimentary rock, schistose metamorphic rock;

▶ Hard crust: carbonate sedimentary rock and metamorphic rock;

▶ Deformation: metamorphic rock cut into large plates;

▶ Bulging: clastic sedimentary rock;

▶ Fracture: all rocks, especially those metamorphic rock with carbonate composition.

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